Header picture doesnt scale

I use the theme twenty thirteen, and I have a problem.. When I upload a picture for the banner, it doesn’t scale. When i go to the website on a full hd screen the banner is correctly, but when i’m on a low res (1366 x 768) screen, only the half of the banner is on the screen.. I tried several resolutions, but that wasn’t the solution… How can I put on a scale modus or something like that? Sorry for my bad Englisch…Site

However, there is a known problem with the Twenty Thirteen (and other themes) which use a background-image property for the header instead of an actual img tag. The banner image gets cropped on either side, so if displaying a full banner image is important to you, then you can look at this post for a solution.